Data communication systems have been under continual development for many years. There is oftentimes difficulty in accommodating the offset, or differences, of the operational frequencies between the various devices within a communication system. Carrier frequency offsets (CFOs) are very common in data communications systems, in that, two or more devices operate at frequencies that are slightly different. When there is a CFO between a transmitter and a receiver within a communication system, there may be significant degradation in performance. The undesirable degradation in performance may result in an inability to demodulate and decode received data. In extreme situations, the existence of CFO may result in a complete inoperability of the communication system.
In order to correct for CFOs, an accurate estimate of the CFO is needed. In packet based communication systems, a known preamble sequence is transmitted with each data packet. The received sequence and the known preamble are typically used at the receiver to estimate the CFO. The prior art includes various methods and approaches that may be used to achieve an estimate of the CFO; however, these prior art methods and approaches typically involve a high degree of computational complexity. This high degree of computational complexity oftentimes prohibits their implementation. In addition, these prior art methods and approaches often fail to provide for a sufficiently high degree of accuracy. The combination of a high degree of computational complexity and relatively low accuracy provides even further motivation for not implementing these prior art approaches. As such, accurate estimates of CFOs may typically not be achieved in most prior art communication systems. This results in a failure to achieve optimal performance and accurate system operation.
When an accurate estimate of the CFO may in fact be achieved, the CFO estimate may then be used at the receiver to correct for the CFO. Prior art approaches to compensate for this CFO, in a receiver, are typically inadequate to provide for sufficient system performance. As with the estimation of the CFO, the prior art CFO correction techniques are also typically extremely computationally intensive and costly. As such, CFO correction if oftentimes not performed at all within a communication system, and a significant degradation of performance is often the result.
One particular type of communication system, a cable modem (CM) communication system, has been under continual development for the last several years. There has been development to try to provide for improvements in the manner in which communications between the CM users and a cable modem termination system (CMTS) is performed. Many of these prior art approaches seek to perform and provide broadband network access to a number of CM users.
CM communication systems are realized when a cable company offers network access, typically Internet, access over the cable. This way, the Internet information can use the same cables because the CM communication system puts downstream data, sent from the Internet to an individual computer having CM functionality, into a communication channel having a 6 MHz bandwidth capacity. The reverse transmission is typically referred to as upstream data, information sent from an individual back to the Internet, and this typically requires even less of the cable's bandwidth. Some estimates say only 2 MHz of bandwidth is required for the upstream data transmission, since the assumption is that most people download far more information than they upload.
Putting both upstream and downstream data on the cable television system requires two types of equipment: a cable modem on the customer end and the CMTS at the cable provider's end. Between these two types of equipment, all the computer networking, security and management of Internet access over cable television is put into place. This intervening region may be referred to as a CM network segment, and a variety of problems can occur to signals sent across this CM network segment. This represents just one of the many situations where a CFO may exist between two different devices, specifically, between the cable modem on the customer end and the CMTS at the cable provider's end.
One particular deficiency that may arise within the CM network segment is the undesirable introduction of a CFO in the expected clock frequency sent from the CMs within the CM communication system to the CMTS. Again, although there do exist some approaches in the prior art to try to estimate this CFO, but these prior art approaches typically fail to provide an efficient solution. As in the general prior art application of trying to perform CFO estimation, these prior art methods and approaches typically involve a high degree of computational complexity. In addition, these prior art methods and approaches often fail to provide for a high degree of accuracy. In order to correct for CFOs, an accurate estimate of the CFO is needed. In packet based communication systems, a known preamble sequence is transmitted with each data packet. The received sequence and the known preamble are usually used at the receiver side to estimate the CFO. However, in many cases, the preamble might not be long enough to obtain the needed accuracy for the CFO estimate. In all of these deficient prior art approaches that seek to perform CFO estimation, any subsequently performed CFO correction is likewise deficient, in that, the prior art CFO correction approach uses a relatively inaccurate CFO estimate.
These CFOs can lead to significant performance degradation in CDMA systems. These offsets can result in loss of orthogonality in the CDMA codes, especially for synchronized CDMA (S-CDMA) systems, which can cause significant inter-code-interference (ICI) that can become a limiting performance factor. The CFO between each two devices is usually estimated using preamble and/or data symbols contained within the transmitted data.
It is noted here that within Time Division Multiple Access (TDMA) communication systems, this problem of correcting for CFO does not exist in the same context of CDMA, as different users' codes are orthogonal even in the presence of CFOs. Thus, any single-user carrier frequency offset correction technique would be adequate within TDMA communication systems.
As such, there exists in need in the art to provide for a more efficient way to perform estimation and correction of CFOs existent within the signals communicated between devices.